This invention is directed to a method for measuring and monitoring damage to structural components within their environment. More particularly, it is directed to a method for measuring damage to structural components caused by stress corrosion cracking.
Exposing structural materials to aggressive environments under steady or cyclic stress can give rise to damage in the form of cracking. This is often referred to as "stress corrosion cracking" or "corrosion fatigue". Stress corrosion cracking of structural materials in aggressive environments is a continuing problem in many industries. It has been particularly so in the nuclear industry where structural materials operate under sustained or cyclic stress in high temperature water, such as that employed in boiling water reactors.
In general, the performance of structural components is predicted in advance from information on the expected loadings and resulting stress from these loadings. Although these predictions are sufficiently accurate to predict service performance, it has been found difficult to predict the lifetime of such performance due to the uncertainty in the environmental conditions and the influence thereof on stress corrosion cracking which results.
An example of the uncertainty of lifetime predictions for structural materials is the stress corrosion cracking which has been found to occur in stainless steel piping used in the nuclear industry. Although designs for new plants attempt to compensate for this phenomenon, it is desirable to monitor and assess the extent of damage in plants which have been operating for a number of years to help predict their lifetimes and possibly extend their lifetime. Methods for assessing the state of damage have been directed towards monitoring the aggressive environment. In the case of boiling water reactors, the water chemistry is measured to determine factors such as resistivity, electrochemical potential, oxygen level and impurity levels. Such measurements are indirect. No direct measurement is made of the effect this water chemistry has on crack growth in the structural materials during plant service. Therefore, the extent to which the lifetime of the structural material is extended by varying operating condition is unknown.
Methods for directly measuring crack growth in specimens removed from their environment have been disclosed over the years. These methods use a variety of monitoring systems including visual and voltage potential drop methods, such as that disclosed by Beevers, Editor, "The Measurement of Crack Length and Shape During Fracture and Fatigue", Engineering Materials Advisory Services, Limited, (1980). However, to measure the crack growth of plant structural components in their environment, an accurate method and apparatus are needed. Accurate measurements of crack growth through potential drop methods have not been obtained previously due to deficiencies in relating voltage measurements to crack size and deficiencies in the equipment utilized. This invention overcomes these difficulties and is capable of measuring crack growth in such environments to within about .+-.0.0001 inch.